The present invention relates to power plants and in particular to a method for optimizing the performance of a power plant cycle.
The production of commercial electric power requires about 30% of the total fuel consumed in the United States. Only about 40% of the available useful work of the consumed fuel is converted into electricity.
Most of this electric power is currently produced in simple reheat or combined Rankine thermodynamic cycles. Although the steam Rankine cycle has been applied worldwide for over seventy years, thermodynamic criteria have been lacking for selecting the working fluid and operating states for even a simple organic binary Rankine cycle for a given set of conditions. Such an organic binary Rankine cycle is utilized, for example, in a typical hydrothermal geothermal power plant wherein the heat source includes one or more reservoirs of geothermal brine which is supplied to the primary side of a heat exchanger of the power plant and an organic fluid, for example, a hydrocarbon, such as isobutane, or isopentane or a mixture thereof is used on the secondary side. The hydrocarbon thus used as a secondary working fluid is heated on the secondary side of the heat exchanger, and passes through the turbine of the power plant for producing electrical power.
The thermodynamic and economic performance of geothermal binary Rankine cycle power plants is influenced by a multiplicity of factors, including resource characteristics, the choice of production methods (i.e. single phase or two-phase brine production), the thermodynamic cycle configuration, subsystem characteristics, fuel cost, subsystem design and off-design efficiency factors, working fluid characteristics, and the selected independent thermodynamic process states. Power plant design is extremely complicated and operational flexibility is extremely limited because of the foregoing factors, yet it is highly desirable that the plant function at or near optimum thermodynamic and economic conditions during its entire operational lifetime. The design process has traditionally been one of multiple iterations, even when highly competent and experienced system designers have had access to powerful state-of-the-art system simulators.
Progress in the commercial exploitation of low to medium temperature goethermal brine resources by organic binary Rankine cycle power plants has been slow because of the foregoing, because resource conditions are highly site specific, and because no general plant design or operational criteria based on practical experience exists. In addition to variations in temperature, salinity, scaling potential, and porosity, hydrothermal geothermal resources obviously vary in physical size (volume) and have different recharge characteristics. Because of porosity, size, and recharge variations, reservoir temperatures will decline with time at various rates depending upon brine production. The variation in temperature of the resource causes power plant peformance to degrade, complicating commercial feasibility and operational decisions which ultimately inhibit exploitation. It has been suggested that degradation in binary cycle performance due to resource temperature decline can be mitigated somewhat by appropriate changes in the working fluid composition. However, when the working fluid is changed the cycle performance changes. Further, theoretical or empirical bases for the selection of optimum working fluids, and independent process thermodynamic states are in their infancy. It is well known that turbine efficiency is severely degraded when operated at extreme off-design conditions. Off-design characteristics of the turbine, therefore, will obviously limit the extent of changes to the working fluid and turbine operating states in response to changing resource conditions.
Accordingly, there exists a need to provide a method for operating a power plant, and in particular a Rankine cycle power plant, to achieve and maintain superior thermodynamic and economic performance.